Shield for nuclear reactor vessel

ABSTRACT

A protective shield for a nuclear reactor vessel is disclosed that is operative to reduce the impact strength of high pressure fluid released as a result of a reactor accident and also to prevent the expulsion of high velocity fragments from the vessel. The shield is constructed of a plurality of independent cylindrical rings that are axialy secured by tie rods. The tie rods are adapted to expand in the event of an inordinate buildup of pressure behind the shield whereby the rings can be displaced to create narrow openings through which the fluid can be released at a reduced rate.

[1 1 3,744,660 1 July 10, 1973Birchall................................... 220/3 F. Bryant, Stephen A.L. Olson, John ABSTRACT Primary ExaminerLeonard Summer AssistantExaminer-James R. Garrett Attorney-Carlton Schneeberger, Eldon H.Luther, Robert F. Carney, Richard H. Berneike, Edward L. Kochey, Jr. andLawrence P. Kessler Gennaro V. Notari, Simsbury, both of Conn.

Cnmbustion Engineering, Inc., Windsor, Conn.

Dec. 30, 1970 a te States Patet [1 1 Gaines et al.

[ SHIELD FOR NUCLEAR REACTOR VESSEL [75] Inventors: Albert L. Gaines,West Simsbury;

[73] Assignee:

22 Filed:

Appl. N0.: 102,800

PAIENIED L 01975 $144,660

SHEET 2 or 2 INVENTORS ALBEQT L. GA/NES ENNARO M NOTAE/ 1 ATTORNEY ISI'lllllElLD FOR NUCLEAR REACTOR VESSEL BACKGROUND OF THE INVENTION Inthe operation of a conventional nuclear power plant heat is transferredfrom the reactivate fuel to a vaporizable fluid which operates as theworking medium in a tube-generator set. The fuel and control elementsare contained in a reactor core that is enclosed by a contaminmentvessel and through which a coolant fluid is passed in heat exchangerelation with the fuel elements to extract heat therefrom. In systems ofthe pressurized water type the coolant fluid is maintained entirely inits liquid phase at high pressures, fluid pressures of 2,500 psi beingnot uncommon, with the generated heat being subsequently passed to avaporizable' fluid in an independent heat exchanger and the resultingvapor employed as the turbine working medium. In boiling water reactorsystems the reactor coolant is maintained at a reduced, yet still high,pressure of about 1,000 psi, being vaporized within the reactor and thevapor passed to the tube-generator set, thereby eliminating the need forindependent heat exchangers.

The extreme fluid pressures encountered in such systems give rise to thedanger of releasing a large quantity of fluid with a tremendous force inthe unlikely event of a failure or rupture of the containment vessel.The forces generated by the rapid expansion of the fluid containedwithin the vessel following fracture of the vessel would not only besufficient to propel any vessel fragments about the environment of theplant as high velocity missiles but could also fracture the fuelelements and their retention structure to likewise propel themexternally of the containment structure. With the fuel elements removedfrom the containment vessel the effectiveness of emergency cooling ofthe elements is reduced or completely lost and the danger of radioactivematerial emmision to the plant environment is created.

It is to the amelioration of the above-cited problem that the presentinvention is directed.

SUMMARY OF THE INVENTION The present invention contemplates providing aprotective shield about a nuclear reactor pressure vessel that willminimize the consequences of a serious reactor accident. The shieldsubstantially completely encloses the vessel thereby to provide anobstruction against the propulsion of high velocity missiles resultingfrom the rapid expansion of the high pressure fluid liberated by ruptureor fracture of the containment structure. It is formed of a plurality ofstacked cylindrical rings that are axially secured in edge-to-edgerelation by means of tie-rods. The tie-rods are free to expand upon thebuildup of fluid pressure behind the shield as would occur upon ruptureof the vessel. Inexpanding, the tie-rods relieve the axial securingforce on the rings such that they can undergo axial movement to createannular spaces therebetween. The spaces so-created enable the containedfluid to be vented externally of the shield and at a reduced rate thatis incapable of producing harmful effects referred to above.

For a better understanding of the invention, its operating advantagesand the specific objects obtained by its use, reference should be madeto the accompanying drawings and description which relate to aparticular embodiment of the invention.

DESCRlPTlON or THE DRAWINGS FIG. 1 is an elevational section of anuclear reactor pressure vessel embodying the protective shield of thepresent invention;

FIG. 2 is a plan view of the apparatus illustrated in FIG. I; and

FIG. 3 is a partial elevational view taken along line 33 of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. I of thedrawings there is illustrated one form of a nuclear reactor vesselarrangement 10 constructed according to the present invention. Itcomprises a generally cylindrical pressure shield 12 whose lower end isclosed by a spherical closure 14 and whose upper end is provided with anannular, radially extending flange 16 for mounting a removable vesselclosure cover 18, the latter being provided with a mating flange 20. Anarray of openings is provided in the closure cover 18 to accommodatepassage of sleeves 21 which receive the manipulating rods of controlelements (not shown) that operate within the reactor core. Attachment ofthe cover 18 to the vessel 12 is achieved by means of threaded studs 22that are circumferentially spaced about the flanges. The interior of thepressure vessel is adapted to contain the reactor core, fuel elements,core support structure and various other associated equipment, none ofwhich is shown for the reason that it is not germain to the presentinvention. A plurality of nozzles 24 are disposed in spaced relationabout the circumference of the vessel 12 and provide means forcirculating high pressure fluid to and from the interior thereof.

According to the invention a protective shield, indicated generally as30, is provided to enclose the reactor vessel 10. The shield 30 operatesto suppress emissions released from the reactor vessel in the event of aserious accident. Due to the unique ability to retard the rate ofexpansion of the high pressure fluid released from the interior of thevessel, it is effective to reduce the impact potential of the forcesgenerated by such expansion. Additionally, the shield serves to containvessel fragments that might otherwise be expelled from the organizationas high velocity missiles under the impulse of the rapid expansion ofthe released fluid.

The shield 30 comprises a plurality of annular, relatively thin walled,cylindrical rings 32 that, in this embodiment, are constructed of formedmetal plate. The rings 32 are vertically stacked and enclose the vesselshell 12 from a level adjacent the upper edge of the lower end closure14 to the lower surface of the shell flange l6. Vertically extendingtie-rods 34 maintain the rings 32 in stacked relation. The tie-rods arethreaded at their opposite ends and secured by the nuts 36 to oppositelyfacing lower and upper flanges, 38 and 40 respectively. Lower flange 38forms an integral part of a generally hemispherical lower end cover 42that encloses the end closure 14 of the shell 12. Upper flange 40 formspart of an annular locking collar 44 that encircles the organizationadjacent the junction of the mating flanges l6 and 20 on the shell 12and its closure cover 18 respectively. The locking collar 44 isvertically supported by an annular shoulder 46 that extends radiallyfrom the inner surface of the collar and bears upon the upper surface ofthe upper shell flange 16.

To accommodate radial passage of the nozzles 24 the adjacent ring 32 isprovided about its upper edge with a plurality of circumferentiallyspaced U-shaped recesses 48 that are formed to receive the undersurfacesof the respective nozzles. The upper, open portions of the recesses 48are closed by a depending annular skirt 50 that is formed integral withthe locking collar 44 extending downwardly from the flange 40 to aboutthe horizontal centerline of the nozzles 24. Inverted U- shaped recesses52 in the lower edge of the skirt 50 conform to the upper surfaces ofthe nozzles. The radial space between the skirt 50 and the adjacent ring32 if filled by a spacer plate 54 formed on its lower edge with a recess56 similar to that formed in the skirt. The spacer plates 54 arethreadedly attached to the ring 32 by means of fasteners 58 whose headsare recessed in the plate.

The closure cover 38 of the vessel organization is enclosed by a domeshaped cover 60 that forms the upper part of the protective shield 30.The cover 60 is provided with openings 61 to accommodate passage of thecontrol rod sleeves 21 and an annular flange 62 that serves to receivethe circumferentially spaced studs 22 that attach it to the vesselflanges l6 and 20 in mutual engagement. The cover 60 is additionallysecured by a plurality of rectangular shear blocks 64 that engage theflange 62 and an annular recess 66 provided in an upwardly extendingportion 68 of the locking collar 44 at circumferentially spaced pointsabout their periphery of the flange 62.

in the protective shield organization 30 illustrated herein the rings 32and the forgings that form the lower end cover 42 and the upper domecover 60 are shown as being sized to be in slightly spaced relation fromthe external surface of the enclosed pressure vessel members. Byfabricating the shield components slightly oversized their assemblyabout the vessel is facilitated in that dimensional inaccuracies of thevessel members can be obviated. it should be understood, however, thatthe shield components can be formed so as to be contiguous with theenclosed members of the vessel without departing from the scope of theinvention.

In assembling the protective shield structure about the vessel the nuts36 that attach the ends of the tierods 34 to the opposed flanges 38 and40 are tightened to prestress the tie-rods ideally to about 40,000 psi.Prestressing the tie-rods in this way places the contiguous edges of therespective rings 32 in compression during normal operation of thereactor such that they form a relatively tight-enclosure thereabout. Inthe event of a serious accident in which the reactor vessel mightfracture or rupture to release its high pressure fluid contents therewill be a buildup of pressure behind the shield structure. The rings 32and forgings 42 and that comprise the shield 30 are sufficiently strongto resist the hoop-tension stresses imposed upon them so as to preventany radial blowout of the members and to contain any high velocityvessel fragments that might otherwise be expelled to the exterior. Thetie-rods 34, however, will be caused to undergo a slight amount of axialexpansion under the action of the forces generated by the high pressurefluid. Expansion of the tierods relieves the compressive loading on therings 32 such that they can be axially displaced thereby creatingannular spaces between the respective rings. It is anticipated that aserious reactor accident of the type involved would result in fluidpressures of about 2,250 psi being established behind the protectiveshield elements. Pressures of this magnitude will generate axial forceson the tie-rods 34 causing them to expand axially about one inch therebypermitting the rings to be axially displaced to create spacestherebetween averaging about one-quarter inch in width. The highpressure fluid will thus be released to the exterior of the shieldthrough the spaces formed between the rings. Because the fluid isreleased through the narrow spaces formed between the rings, the rate ofits expansion will be retarded thus operating to eliminate or minimizethe danger of its creating high impact forces capable of causing seriousdamage to the containment structure of the plant or to its surroundingsas could otherwise be created were the fluid uncontained and therebyfree to expand rapidly.

It will be understood that various changes in the details, materials,and arrangements of parts which have been herein described andillustrated in order to explain the nature of the invention, may be madeby those skilled in the art within the principle and scope of theinvention as expressed in the appended claims.

What is claimed is:

1. In combination, a generally cylindrical pressure vessel forcontaining a fluid under pressure and having a wall thickness to providesufficient structural strength to withstand the highest expectedpressure of said fluid and a protective shield enclosing said vessel,said shield comprising:

a. a plurality of cylindrical members concentrically disposed withrespect to said vessel in stacked endto-end relation and in radiallyspaced relation to said vessel.

b. longitudinally extending tie means for imparting a compressiveloading upon said cylindrical members and disposing them in mutuallycontiguous relation when said pressure vessel is intact,

c. said cylindrical members and said tie means being free from exposureto stresses imposed by the fluid contained in said vessel when the sameis intact, and

d. said tie means being of insufficient structural strength to withstandthe pressure of the fluid within said vessel and being expandable in adirection parallel to the axis of said cylindrical members upon ruptureof said pressure vessel in an amount to provide spaces between saidcylindrical members for the release of liberated fluid.

2. The combination recited in claim 1 wherein said cylindrical memberscomprise discrete members defining a generally cylindrical bodyenclosing said vessel at least along a substantial portion of its axis.

3. The combination as recited in claim 2 wherein said cylindricalmembers are formed of hollow, open cylinders having a wall thicknessless than that of said pressure vessel.

4. The combination as recited in claim 2 including a pairof axiallyspaced members concentrically disposed about said vessel adjacent theopposite ends thereof, said members bearing annular flanges attachingthe ends of said tie means, at least one of said members engaging saidrings for imparting a compressive load thereon upon tensioning said tiemeans.

5. The combination as recited in claim 4 wherein said tie means comprisea plurality of tie rods circumferentially spaced about said flanges.

8. The combination as recited in claim 7 wherein one of said flangebearing members is a generally cylindrical locking collar compressivelyengaging said pressure vessel flange, a dome shaped closure coverenclosing the upper end of said pressure vessel, and shear block meanscooperating with said locking collar for securing said closure cover inassembled relation with said pressure vessel.

1. In combination, a generally cylindrical pressure vessel forcontaining a fluid under pressure and having a wall thickness to providesufficient structural strength to withstand the highest expectedpressure of said fluid and a protective shield enclosing said vessel,said shield comprising: a. a plurality of cylindrical membersconcentrically disposed with respect to said vessel in stackedend-to-end relation and in radially spaced relation to said vessel, b.longitudinally extending tie means for imparting a compressive loadingupon said cylindrical members and disposing them in mutually contiguousrelation when said pressure vessel is intact, c. said cylindricalmembers and said tie means being free from exposure to stresses imposedby the fluid contained in said vessel when the same is intact, and d.said tie means being of insufficient structural strength to withstandthe pressure of the fluid within said vessel and being expandable in adirection parallel to the axis of said cylindrical members upon ruptureof said pressure vessel in an amount to provide spaces between saidcylindrical members for the release of liberated fluid.
 2. Thecombination recited in claim 1 wherein said cylindrical members comprisediscrete members defining a generally cylindrical body enclosing saidvessel at least along a substantial portion of its axis.
 3. Thecombination as recited in claim 2 wherein said cylindrical members areformed of hollow, open cylinders having a wall thickness less than thatof said pressure vessel.
 4. The combination as recited in claim 2including a pair of axially spaced members concentrically disposed aboutsaid vessel adjacent the opposite ends thereof, said members bearingannular flanges attaching the ends of said tie means, at least one ofsaid members engaging said rings for imparting a compressive loadthereon upon tensioning said tie means.
 5. The combination as recited inclaim 4 wherein said tie means comprise a plurality of tie rodscircumferentially spaced about said flanges.
 6. The combination asrecited in claim 5 wherein one of said flange-bearing members is a domeshaped cover enclosing the lower end of said vessel and including anupper surface engaging the lowermost ring member.
 7. The combination asrecited in claim 6 wherein said pressure vessel includes an annularflange integrally formed adjacent the upper end thereof, said flangecompressively engaging the uppermost cylindrical member.
 8. Thecombination as recited in claim 7 wherein one of said flange bearingmembers is a generally cylindrical locking collar compressively engagingsaid pressure vessel flange, a dome shaped closure cover enclosing theupper end of said pressure vessel, and shear block means cooperatingwith said locking collar for securing said closure cover in assembledrelation with said pressure vessel.